In a typical IVF or ICSI treatment cycle, the woman’s ovaries are stimulated to produce many eggs. Following fertilisation and embryo culture, the best embryos are selected for embryo transfer. For about 50% of couples, there will also be good embryos which are surplus to those required for embryo transfer. These embryos can be frozen at this point for future use.

Embryo freezing (cryopreservation) is a method of preserving the viability of embryos be carefully cooling them to very low temperatures (-196oC). This is carried out in the laboratory using specialised freezing equipment and the embryos can then be safely stored in liquid nitrogen for extended periods.

What are the benefits of embryo freezing?

The main benefit of embryo freezing is the option to have frozen embryos thawed and transferred to the woman’s uterus in the future without having to undergo stimulation of the ovaries or egg retrieval. It is also possible that there may be enough frozen embryos for more than one subsequent cycle.

What does a frozen embryo transfer cycle actually involve?

Frozen embryo transfer cycles are relatively simple. In some cases, the woman’s natural cycle will be monitored by ultrasound to assess the development of the lining of the uterus and to determine the timing of ovulation and hence embryo transfer. In other cases, depending on the woman’s history a more hormonally controlled cycle may be warranted. The decision regarding the most appropriate treatment will be made in consultation with the clinician.

Are there any disadvantages to embryo freezing?

Approximately 70% of the embryos that are frozen will survive the thawing process. This does however vary between patients and it is possible that none of a couple’s embryos will survive the freezing and thawing process.

How successful are cycles involving the transfer of frozen / thawed embryos?

Overall, frozen embryos have a slightly lower chance of implanting than fresh embryos. This is mainly because in general the best embryos in a group will already have been transferred on the fresh embryo transfer.